The brain is well-known to be highly electrically active at all times, as evidenced by measurements on a subject's scalp in an electroencephalogram reading. This electrical activity can be synchronized with an event such as the occurrence of a discrete sound, as well as more elaborate stimuli such as a semantic change in a spoken phrase. This synchronization can be defined as an evoked potential, or evoked response, and recorded from the nervous system of a subject as an electrical potential following presentation of a stimulus. These evoked potentials are distinct from spontaneous potentials as detected by electroencephalography or electromyography.
Traditionally, transient response stimulus and analysis methods are employed to analyze auditory evoked potentials and provide a tool by which to monitor peripheral and central auditory function, assess its maturation, and overall obtain a global view of the integrity of the auditory pathway. However, there is a fundamental shortcoming of this conventional approach by virtue of the way the auditory evoked potentials are traditionally analyzed.
Conventional transient auditory evoked potentials are analyzed via signal averaging, which is performed in an effort to improve the signal-to-noise measurement embodied in a given response. However, the stimulus-related portion of the response is only known in general form from the examiner's subjective reference. Even after substantial signal-to-noise improvement, the putative response must be judged in a background of residual noise from which it can be difficult to distinguish some, if not all, of the component waves which make up the auditory evoked potential. In short, transient evoked potentials do not allow for the evoked response to be predicted, with a sufficient degree of accuracy, by the stimulus administered. Consequently, auditory evoked potential response analysis can be strongly based on examiner judgment, thus introducing the element of subjectivity to any analysis and conclusions drawn.
Furthermore, a particular challenge for researchers and clinicians alike are the potential ambiguities of wave identification over the course of maturation. In other words, the time it takes for transient auditory evoked potentials to reach well-defined adult waveforms is dependent upon structural and functional development, maturational effects of myelination, synaptic density and neuro-plasticity of the neural pathways specific to individual auditory evoked potential components. For lower sites along the auditory pathway (i.e., namely pontine-ward) development is relatively short, for example, the auditory brainstem response reach maturity by approximately two years of age. Conversely, for higher sites along the auditory pathway (i.e., namely cortical-ward) development is relatively long (which reflect middle latency response) and reach maturity by 12 years of age. Similarly, late-cortical components (which reflect long latency response) reach maturity by 17 years of age. Of particular concern is the potentially confounding maturational and pathological changes present in auditory evoked potentials, and thus potential hazards of using age-corrected norms. Moreover, even in adults, pathological changes via central nervous system disorders and trauma differentially affect auditory evoked potentials further, if not confound precise wave identification, especially with the diversity of focal injuries that can occur.
Accordingly, there is a need for a shift toward a more analytical approach to the auditory evoked response, such as the steady-state response provided in the present invention. Additionally, such an approach can further provide an alternative view of the brain's responses to auditory stimulation and can serve such interests as tracking maturational changes and/or effects of brain disease or injury, when extended to incorporate a more comprehensive representation of the auditory evoked potential component waves, namely to cover the latency range of traditional auditory evoked potentials accessed via a transient stimulus-response analysis.